Microchimerism is a phenomenon in which an organism's body contains a small number of cells from another organism. Unlike tetragametic chimerism, microchimerism is acquired. In tetragametic chimerism, on the other hand, an inborn trait occurs when two non-identical zygotes or blastocysts fuse prior to implantation and develop into a single organism composed of two cell populations, each with its own distinct genes. Microchimerism occurs in many species, including humans.
Microchimerism in humans commonly happens during pregnancy. Cells from the fetus' immune system can enter the mother through the placenta, where they may survive and continue to reproduce through mitosis. In some cases, the descendants of these fetal cells remain in the mother decades later. The same process in reverse can result in populations of maternal immune cells living in the fetus, though this is less common. It can also result from organ transplants and blood transfusions.
There is some research indicating that microchimerism may cause some autoimmune disorders, such as lupus. The presence of maternal immune cells has been blamed for some autoimmune diseases suffered by children, such as juvenile dermatomyositis. Some cases of systemic sclerosis, a disease that damages the skin, joints, and some internal organs, may be caused by fetal immune cells living in the mother, and some studies have linked the presence of fetal immune cells with breast cancer. Autoimmune disorders are more common in women than in men, and the effects of fetal cells have been proposed as a possible reason for this.
The implications of research in this area are still ambiguous, however. The frequent presence of fetal or maternal immune cells in tissues affected by these diseases may indicate that they cause or contribute to the disease, but it could also mean that the foreign cells are present because they are helping to fight the disease or mitigate the damage. Thus, it has also been hypothesized that some microchimerism may actually be beneficial.
In some animals, such as cattle, it is common for placentae to join together in the womb and share blood circulation. As a result, cells can easily pass between fraternal twins in the womb, resulting in microchimerism. In the case of a male and female fetus joined in this way, the interchange of sex hormones during development causes the female to become partially masculinized. This prevents normal development of the reproductive organs and results in what is known as a freemartin, a sterile female who may have partially masculine characteristics. The presence of microchimerism is used to confirm that a female is a freemartin, as her blood will contain cells with male genetic material that originated in her twin.